Voltage limiter



March 7, 1939. H. E. VAUGHAN 2,149,372

VOLTAGE LIMITER Filed April 13, 1938 :1 C) g i 35 E as [g 20 as l'll'v /zb/ lNl/ENTOR H. E VAUGHAN grams);

Patented Mar. 7, 1939 PATENT OFFICE.

2,149,372 von'rnca IJMITER.

. Henry E. Vaughan, Valley Stream, N. Y., assignor to Bell Telephone laboratories, Incorporated, New York, IN. Y., a corporation of New York Application April 13, 1938, Serial No. 201,714

2 Claims. (Cl- 179-78) This invention relates to voltage limiting apparatus, and more particularly to an arrangement including gaseous discharge devices for limiting voltages in alternating current transmission systems.

Voltage limiting circuits employing gaseous discharge devices have been used heretofore to protect alternating current transmission systems from excessive voltages. One such circuit is disclosed in the patent of A. M. Curtis No. 1,937,143

issued November 28, 1933, and comprises a pair of gaseous discharge devices each having a cathode, anode and control electrode and connected between the two branches of the system suchthat the control electrodes and ,cathodes provide bridging paths of opposite polarity for shunting excess current between the branches when discharge is instituted in the gaseous devices by voltages applied across the anodes and cathodes thereof.

A control transformer having its primary winding connected in shunt of the two branches and the secondary windings associated therewith and applied across the respective anodes and cathodes serves to supply voltage for instituting discharge in one or in both gaseous devices.

- induced in the associated secondary winding and applied across the respective anodes and cathodes are below the amount required to maintain discharge. Hence the maintenance of the discharged condition must be placed under control of the control electrode of each gaseous device.

The present invention contemplates a voltage limiting arrangement employing gaseous 'discharge devices and in which discharge in the latter is not only instituted by a voltage applied across the respective anodes and cathodes but is also maintained by the voltage impressed thereon.

in a preferred embodiment the invention comprises two impedances one of which is disposed in each branch of the system between the points to which are connected the primary winding of a control transformer and the control electrodes of a pair of gaseous devices connected to provide two bridging pathsacross the system in the manner aforementioned. As discharge in one or in both gaseous devices is instituted both impedances are included in each bridging path to cause in secondary winding of the control transformer the induction of a voltage that, when applied across the respective anodes and cathodes, is sufiicient to maintain discharge so long as a voltage above normal value is present in the system. According to this invention, therefore, the primary winding of the control transformer is not short-circuited during discharge as both impedancesare connected in the bridging pathsregardless of the polarity of the voltage causing the discharge.

This invention will be more readily understood from the following description taken together with the accompanying drawing, the single v figure of which is a diagrammatic circuit illustrating the preferred embodiment thereof.

Referring to the drawing a source l0 supplies waves of alternating current voltage through a transmission system comprising branches H and I2 across which are bridged primary windings l5 and IQ of a control transformer whose secondary windings l6 and 20 are applied across the anodes and cathodes of the respective gaseous discharge devices l8 and 22. The associated windings l5 and I6 step up the voltage for one-half of waves drawn from the system while the associated windings l9 and 20 step up the voltage for the other half of waves also drawn from the system. The value of the combined impedance of the primary windings l5 and I9 is such that substantially none of the current in the normal transmission range is shunted across the branches Hand l2.

Each of the gaseous devices is preferably of a type described in the patent of A. M. Curtis, supra, and embodies an anode 5, a cathode 6, a

control electrode 1 and a cathode heater 8. Also,

each gaseous device has its control electrode connected to one branch and its cathode to the opposite branch so that a bridging path is provided thereby for shunting excess current across the system when discharge in instituted in a manner that will be subsequently explained. The bridging paths are normally interrupted through the gaseous devices during the transmission of voltages falling within a normal range.

The operation of the above voltage limiter is briefly as follows:

As voltages of a normal range are being transmitted in the system no shunting action takes place due to the infinitely large impedance of each gaseous device. During such intervals, however, the relatively high combined impedance of the primary windings l5 and I9 permits small amounts of current to flow therethrough so that the voltages induced in the respective secondary windings l8 and II are of a magnitude less than one or both of the gaseous devices.

As the voltages in the system rise above the normal range the current flowing in the primary windings I5 and I 9 increases until eventually voltages induced in either secondary winding i6 or secondary winding 20 or both attain a value equal to at least the discharge voltage of the respective gaseous devices. As a result one or both of the gaseous devices is discharged depending on the polarity of the applied wave. The discharge produces a flow of space current between the respective control electrodes and cathodes to establish a bridging path between branches II and I 2 whereby excess current is shunted directly across the system. This shunting of current insures that the voltage applied across the primary winding of a step-up transformer 34 will be limited to at least a predetermined maximum value notwithstanding that voltages abovethe normal range are present in branches I I and I2.

In discharging, the initial infinite impedance of the control electrode-cathode path of each gaseous device is considerably reduced so that in effect a substantially dead short circuit is established across the primary windings I5 and I9. As the impedance of the primary windings I5 and I9 is relatively high as compared withthe reduced impedance of the bridging path extending through the discharged gaseous device very little current will fiow through the primary windings I5 and I9. Hence any voltage now induced in either secondary winding I6 or secondary winding 20 will be relatively small and below that amount required to maintain a discharged condition in the gaseous devices. Consequently, it is necessary to maintain the discharged condition under control of the voltages impressed on the respective control electrodes. As pointed out in the patent of A..M. Curtis, supra, a voltage of the order of three volts impressed on a control electrode is suflicient to maintain the discharge conditicn and therefore'so long as this voltage is supplied the system will .be protected against excessive voltages. when, howeven-the voltage applied to either control electrode falls below-three volts, the gaseous device associated therewith will return to the undischarged condition and thereby terminate the flow of currentin the bridging paths established as described above.

' In accordance wih the present invention the "discharge of gaseous devices utilized in voltage limiting circuits of the type hereinbefore described is not only instituted but also maintained by voltages applied across the respective anodes andLcathodes thereof insteadof by the voltage" impressed on the control electrode as in the similar circuit disclosed in the patent of A. MfCurtis, supra,

To eflectuate both the institution and main-- tenance of discharge under control of anodecathode voltages an impedance 30 of the order of- 10,000 ohms is interposed in the branch II between the points to which are connected one end of the primary winding I5 and the control electrode of gaseous device I8, and an impedance 3| of similar order is interposed in branch I2 between the points to which are connected the opposite end of the primary winding I9 and the control electrode'of gaseous device 22. Also an impedance 32 of the order of 100,000 ohms is cone nected in theanode-cathode circuit of the gaseous device I8 while a similar impedance 33 is connected in the anode-cathode circuit of gaseous .device 22. The impedances 32 and 33 are that required to break down or discharge either respective anode-cathode circuits. Batteries 23 and 24 are utilized to bias the control electrodes of the gaseous devices I8 and 22, respectively. so as to prevent discharge in any manner other than by. that which will be hereinafter explained.

The secondary winding of the coupling transformer 34 is connected across the input of thermionic amplifier 35 whose output is connected to a load 36. Battery 31 suppliesplate potential for this amplifier while battery 38 energizes the heaters associated therewith and also those associated with the gaseous devices I8 and 22. P0- tentiometer 39 bridged across the input of the thermionic amplifier provides an adjustment for controlling the gain thereof. Resistance 40 and condenser 4i connected in series across the secondary winding of the transformer 35 serves to even oi! the frequency characteristic of the thermionic amplifier.

Assuming current of requisite polarity flowing in primary winding I5 so as to induce in its associated secondary winding I6 a voltage which is at least adequate to institute discharge of gas-.

eous device I 8, then current will flow in a bridge circuit comprising branch II, impedance 30, .control electrode and cathode of gaseous device I 8, lead 42, impedance 3| and branch I2. Since the primary windings I5 and I9 and the impedances 30 and 3| are connected in parallel, the voltage developed across the impedances 30 and 3| will also be applied across the primary windings I5 and I9. Now, the bridging paths including the impedances 30 and 3| supplant the substantially deadf short circuit that was established across the primary windings I5 and I9 in the prior art circuit hereinbefore described. Therefore, a portion of the current that would flow in the bridging path in accordance with the prior art circuit will now be diverted into windings I! and I9.

This additional current causes in associated secondary winding IS, the induction of a voltage of the samepolarity that initially caused the disin the secondary I6 the induction of a voltage which is at least equal to the discharge voltage of the gaseous device I8, this condition will be maintained. When, however, the current of requisite polarity flowing .in the branches II and I2 falls to the normal range, current flowing in the bridging path and primary winding I5 will be correspondingly reduced. Hence the voltage induced in the associated-secondary winding I6 will be reduced to the cut-off level of the gaseous device I8 to discontinue the discharge thereof. Obviously, when this occurs the bridging path mentioned above is interrupted and transmission in the system again proceeds on a normal basis.

proportioned to limit the current flowing in the extending through the gaseous device II to cause Assumin'ga current of requisite polarity flowing path shunting current across the system devel-,

ops across impedances l0 and 3i a voltage that will increase the .flow of current of requisite polarity in primary winding I! in the manner hereinbefore mentioned relative to a gaseous device l8. Hence the increased voltage induced in associated secondary winding 20 maintains the gaseous device 22 in the discharged state. When the current of the polarity that caused the initial discharge of gaseous device 22 falls to within a normal range, this gaseous device will return to the undischarged state and, 01' course, the bridging path extending therethrough will be interrupted. Thereafter transmission will proceed on a normal basis.

It is seen therefore that impedances ll and ii together with the primary windings II and I9 and associated secondary windings l8 and -20, cause anode-cathode voltages that both institute and maintain the discharge of the gaseous devices regardless of the voltage impressed on the respective control electrodes. In the arrangement according to this invention, minimum amounts of anode-cathode voltageof opposite polarity required for maintaining discharge are attained before the minimum amounts of voltage applied to the control electrode for the same purpose are reached. Consequently, the present invention provides an extremely sensitive and quick-acting voltage limiting arrangement for protecting an alternating current transmission system from excessive voltages.

Amplifier 35 is utilized to compensate for whatever losses occur in impedances 30 and ii when the invention is employed in asignaling transmission system and may be omitted when the invention is otherwise utilized.

It is understood that the invention is capable of modifications other than that disclosed herein and the scope thereof together with such modifications is defined in the appended claims.

What is claimed is:

1. In an alternating current transmission system including in combination means for protecting the system from excessive voltages of either polarity comprising a pair of gaseous discharge devices each having a cathode, anode and control electrode and connected in the system such that the control electrodes and cathodes provide bridging'paths of opposite polarity for shunting excess current across the system when discharge therein is instituted by an adequate voltage impressed on the respective anodes and cathodes, a circuit connected in shunt oi the system for receiving a voltage therefrom, means for coupling the circuit to the anode and cathode of each gaseous device to apply thereto a higher voltage than is received from the system so as to institute discharge in one or in both gaseous devices as the voltage in the system rises above a normal value; means included in the bridging paths and responsive to the current flowing therein for supplying voltage to the circuit and thereby to the coupling means to maintain discharge in one or in both gaseous devices so long as the voltage in the system continues in excess of a normal value.

2. The alternating current transmission system according to claim 1 in which the last-mentioned means is an impedance connected in each branch of the system between the points to which the circuit and control electrodes are connected.

HENRY E. VAUGHAN. 

